feat: add attachment preview UI component (#66)

- Create Attachment interface in messages.ts
- Create AttachmentPreview.svelte component with:
  - Image thumbnails for image attachments
  - File icons for documents
  - File size display
  - Remove button on hover
- Add attachments array to Conversation interface
- Add attachment management methods to conversation store:
  - addAttachment, removeAttachment, clearAttachments, getAttachments
- Integrate AttachmentPreview into InputBar
- Clear attachments on conversation reset

src/lib/stores/claude.ts

@@ -25,6 +25,7 @@ export const claudeStore = {
   isProcessing: conversationsStore.isProcessing,
   grantedTools: conversationsStore.grantedTools,
   pendingRetryMessage: conversationsStore.pendingRetryMessage,
+  attachments: conversationsStore.attachments,
 
   // New conversation-aware subscriptions
   conversations: conversationsStore.conversations,
@@ -67,6 +68,12 @@ export const claudeStore = {
   saveScrollPosition: conversationsStore.saveScrollPosition,
   getScrollPosition: conversationsStore.getScrollPosition,
 
+  // Attachment management
+  addAttachment: conversationsStore.addAttachment,
+  removeAttachment: conversationsStore.removeAttachment,
+  clearAttachments: conversationsStore.clearAttachments,
+  getAttachments: conversationsStore.getAttachments,
+
   getGrantedTools: (): string[] => {
     let tools: string[] = [];
     conversationsStore.grantedTools.subscribe((t) => (tools = Array.from(t)))();
@@ -86,6 +93,7 @@ export const claudeStore = {
     conversationsStore.setWorkingDirectory("");
     conversationsStore.setProcessing(false);
     conversationsStore.revokeAllTools();
+    conversationsStore.clearAttachments();
     // Also clear history restoration
     clearHistoryRestore();
   },